riscv/signal: Fixup additional syscall restarting

The function of do_notify_resume called by entry.S could be entered
in loop when SIGPENDING was setted again before sret. So we must add
prevent code to make syscall restart (regs->sepc -= 0x4) or it may
re-execute unexpected instructions.

Just like in_syscall & forget_syscall used by arm.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/signal.c

@@ -234,6 +234,9 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 
 	/* Are we from a system call? */
 	if (regs->scause == EXC_SYSCALL) {
+		/* Avoid additional syscall restarting via ret_from_exception */
+		regs->scause = -1UL;
+
 		/* If so, check system call restarting.. */
 		switch (regs->a0) {
 		case -ERESTART_RESTARTBLOCK:
@@ -272,6 +275,9 @@ static void do_signal(struct pt_regs *regs)
 
 	/* Did we come from a system call? */
 	if (regs->scause == EXC_SYSCALL) {
+		/* Avoid additional syscall restarting via ret_from_exception */
+		regs->scause = -1UL;
+
 		/* Restart the system call - no handlers present */
 		switch (regs->a0) {
 		case -ERESTARTNOHAND: